Probably an unpopular opinion, but I would have loved a post-Calling All Stations album by Genesis with Ray Wilson. While it's certainly not their best album, I really like the direction they were heading in, and I really like Ray's solo output since. I think another album or two would have given us some great music and given Ray a bit more recognition. And without it, CAS seems like an afterthought for the band, which is a shame because it is really good.
...And obviously, although Jason is my favorite Metallica member, I've always wondered the direction Metallica would have gone if Cliff was still alive.